CDMA is the second most widely deployed technology in the world with more than million subscribers worldwide and is projected to reach.
Table of contents
The receiver architecture is far more complicated and a critical component that is completely left out of this huge expensive book. Another complaint, this book is titled "Designing cdma Systems". To design a system one must understand both standards and theory. This book devotes a lot of pages to describing the standards which can be found in freely available standards documents.
But very little to theory. Chapters 1 and 3 contain good theoretical content. However, this is a mere 30 pages. Overall, this book touches on the easy parts of CDMA, leaves out the hard parts, and fills the rest with rehashed standards documents. The result is a massive book that costs far more than it is worth. Convolutional code [ 5 ] is a forward error correction FEC code, which allows a receiver to recover the corrupted received data by exploiting the redundancy in the encoded data stream. Code symbol outputs from the convolutional encoder may be repeated based on the repetition factor.
The repetition factor represents the number of times that each symbol appears right after symbol repetition. For instance, a repetition factor 2 means a symbol is repeated once. Symbol repetition plays a role in adjusting the transmission rate.
Following symbol repetition, some symbols are punctured or deleted as required to provide a correct rate for the modulation process. The puncturing patterns used with convolutional codes are defined in [ 1 ]. After puncturing, the symbol ordering is rearranged in the block interleaver. This is to protect groups of data from being corrupted at the same time by any deep fade or noise burst. Details of interleaver parameters for different sizes have been specified in [ 1 ]. Following the block interleaver, symbols may be repeated for further rate adjustment.
As an example, Table 2 lists the parameters associated with each block in Fig. Please refer to [ 1 ] for further details on the parameters in each block. This section describes first the manner in which the reverse channel signals are spread and combined, and then the arrangement of quadrature spreading.
Figure 5 illustrates the modulation process for Radio Configurations 3 through 6.
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The long code generator, which will be discussed in Sec. For Radio Configurations 1 and 2, the procedure is similar to that of IS and is not described here. Orthogonal codes comprise a set of chip sequences in which each sequence is orthogonal to the others. Orthogonal codes, also known as canalization codes, are used to ensure orthogonality e. Each channel is spread by an orthogonal code defined by a Walsh function [ 6 ]. The orthogonal characteristics of Walsh functions ensure that the channels do not interfere with each other. The Walsh functions are row vectors of Walsh-Hadamard matrices.
The specific Walsh functions used for different reverse channels are listed in Table 3 As shown in Fig. Further details of Walsh code assignments for traffic channels along with allowable data rates and corresponding post-interleaver repetition rates can be found in [ 1 ]. The reverse channels spread by the orthogonal codes are combined to make up a complex sequence. The real and imaginary parts of the complex sequence are referred to as the I-Channel and Q-Channel, respectively.
Each reverse channel is scaled by a relative gain before the combination. The relative gains will be discussed in the link budget calculations in Sec. The complex sequence is then multiplied by the quadrature spreading sequence that will be discussed in the next section.
Although reverse traffic channels are intended not to interfere with each other by orthogonal codes, in practical situations they can not maintain their orthogonalities due to the effect of filtering as well as multipath fading. Multipath fading is a propagation phenomenon of the channel variations characterized by the arrival of multiple versions of same signal due to reflection, diffraction and scattering of radio waves. The quadrature spreading sequence is arranged in a way that reduces the effect of the multipath fading and restores some of the orthogonality losses between users.
The arrangement of the quadrature spreading is shown in Fig.
CDMA System Design
The long code generator is required to generate spreading sequences at a chip rate of 1. As shown in Fig. This is to ensure that cross correlations between the signals from distinct stations are always small. For spreading rate 1, the PN sequences are based on the following generator polynomials:. The starting position of the Q-Channel sequence is the starting position of the I-Channel PN sequence delayed by 2 19 chips.
Following quadrature spreading, the resulting complex impulse sequence is passed through a transmission filter to avoid interference with adjacent frequency bands [ 1 ]. The propagation channel model used in reverse link systems is that specified by IMT 7 for Vehicular model-A 8. This model takes into account both the slow and fast fading.
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The slow fading is long-term variations in the received signal level and its envelope is modeled by a lognormal distribution [ 7 ]. The slow fading is also referred to as shadowing. The fast fading is short-term variations in the received signal level and is modeled by the superposition of multiple paths with different average powers and arrival times. The average power and arrival time are assumed to be fixed and are determined by the channel impulse response.
Each path has a Rayleigh distribution, with the power spectrum suggested by Jakes [ 7 ]. Figure 6 shows a six-path frequency selective fading channel that has been used for the reverse link. Further details on the channel modeling can be found in [ 4 ]. After the fading channel, as shown in Fig. Note that the dominating interference in CDMA systems tends to be inter-cell interference which is mainly due to the system having a unity reuse 9 factor if traffic is very heavy in adjoining cells.
In other words, the aggregate energy of the interference from neighboring cells may be higher than the energy of the desired signal. The receiver for the reverse link model consists of a rake receiver followed by a channel despreader. Figure 7 illustrates the block diagram of the receiver. The rake receiver attempts to collect the dispersed signal energy resulting from the multiple propagation paths between the transmitter and receiver.
The rake receiver can therefore significantly reduce the effect of multipath fading. The output of the filter is then fed into the rake receiver. The rake receiver consists of several rake fingers and a combiner. Since the received signal is a composite signal made up of delayed versions of the transmitted signal with different attenuations, each rake finger is intended to focus on one of the multiple paths in the demodulation process.
In this way, the rake receiver can create the output with a higher signal-to-noise ratio by combining the outputs of the rake fingers as compared with a single finger, i. It is important for the rake receiver to estimate the channel coefficients e. For the purpose of comparison, we also developed an ideal rake receiver, which assumes that the perfect precise channel information is available at the receiver. Since the ideal receiver can provide the best achievable performance, it can be used to evaluate and measure the relative performance of any practical non-ideal rake receiver.
Details of designing the rake receiver will be discussed in succeeding sections. The receiver may detect and combine up to N replicas of transmitted signals by using a rake receiver with N rake fingers. The number of available paths replicas of the transmitted signals at the receiver depends on the bandwidth of the transmitted signal as well as the characteristics of the propagation channel because the signal with a larger bandwidth has higher time resolutions.
Therefore, spreading rate 3 can generally have a larger number of resolvable paths at the receiver than spreading rate 1. We designed a rake receiver with four fingers for spreading rate 1 1.
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Figure 8 illustrates the rake finger structure. Each rake finger exploits the unmodulated pilot signal on the R-PICH to estimate the channel coefficients. Since the pilot signal is known at the receiver, the channel coefficients can be estimated by simply removing the spreading sequences quadrature spreading sequence and Walsh sequence for the R-PICH. The pilot signal is also used to adjust the timing to track the aimed path see Sec. The rake receiver uses the complex conjugate values of the channel coefficients when combining the outputs from the fingers.
A method known as maximum ratio combining is used to maximized the signal-to-noise ratio, provided that the noise including interference at each finger is an independently and identically distributed Gaussian random variable. Since R-PICH carries the unmodulated signal spread by the long code, the output of the long code despreader can be used to estimate the channel coefficients at each rake finger. It is important to estimate the channel as accurately as possible to increase the signal-to-noise ratio effectively when combining the outputs of rake fingers.
There are two popular approaches to enhance the accuracy of estimation. One is to increase the signal-to-noise ratio by using a longer observation duration. The other is to apply interference cancellation to suppress multipath as well as inter-cell or intra-cell interferences. Currently, there are a number of interference cancellation techniques that can be utilized for CDMA systems. This is because the interference in cdma cannot be considered as a cyclostationary process i. However, methods such as ZF zero forcing equalizer [ 8 ] may found to be more suitable if all the delays of the paths can be accurately estimated.
Nevertheless, such a method would suffer from high computation cost e. In our simulation model as shown in Fig. At the same time, at higher velocities high mobility where the channel changes dramatically, a shorter length filter should be deployed.
In addition, depending on the channel variation, the filter taps g 0 , g 1 , …, g K should also be selected in accordance with channel conditions. The performance evaluation of such filter under various testing conditions will be discussed in Sec. Since the propagation channel is time-varying in general, the rake finger need to keeps track of its change adaptively.
Figure 10 illustrates the delay adjustment block. The pilot symbols are used to adjust the delay at each finger. As shown in this figure, within each finger there are three paths, an early path, an on-time path and a late path, which are correlated with the same long code with slightly different phases starting points. In each of these paths the pilot Walsh sequence is removed and then integrated over the symbol duration.
The magnitudes of the outputs are used to calculate a timing correction once per frame. At the end of the frame, the delay adjustment block evaluates the following value,. Looks like you are currently in Russia but have requested a page in the United States site. Would you like to change to the United States site?
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CDMA technology is complex to design due to its inherent adaptive characteristic and the introduction of data requires a complete new way of analysing the network from traffic characteristics to performance requirements. The authors bring a wealth of experience in developing solutions for wireless design at CelPlan Technologies, Inc.
They followed up the evolution of the wireless technology providing innovative solutions at each step.
In this book they summarize the description of the CDMA technology, revisit basic design concepts and propose new solutions to design and optimise these complex networks. Designing CDMA Systems is highly relevant for engineers involved in the design or operation of CDMA systems, as well as providing a broad understanding of the area for researchers, professors and students in the field.
He is responsible for the design of several wireless networks in South America and for training classes on planning, deployment and commissioning of wireless equipment infrastructure. He planned and supervised the design of nationwide telecommunication networks, covering transmission, switching and wireless elements.